ON GOLD AND SILVEH. 513 



The greater part of the more productive auriferous veins are contained 

 •within Cambrian or Silurian rocks, generally in argillaceous strata or in 

 alternations of slates and thin sandstones. But some veins are in 

 Archgean rocks (S. America, W. of Lake Superior, and India) ; some in 

 altered rocks, which are supposed to be of Triassic, Jurassic, or Cretaceous 

 age. These newer rocks occur along mountain chains, where the beds 

 have been greatly disturbed, folded, contorted, and faulted, and where 

 rocks of very different ages occur close together. There are, therefore, 

 frequently difficulties in deciding the exact age of gold-bearing rocks ; 

 but at present the evidence appears to be in favour of a great part of 

 the rocks with veins of auriferous quartz along the western side of North 

 and South America being of Secondary age. 



The age of the rocks containing the veins does not decide the age of 

 the auriferous veins themselves. Some veins of gold-quartz traversing 

 the Archeean rocks of North America are pre-Silurian, because a con- 

 glomerate at the base of the Silurian rocks in Dakota contains gold ; and 

 also because in Canada the Silurian limestones rest horizontally upon the 

 denuded edge of the Archaean rocks and of the auriferous quartz-veins. 

 The Geological Survey of Canada is now engaged in mapping these 

 areas ; tracing the boundary of the Silurian limestone is important here 

 in limiting the areas within which gold may be looked for. The aurife- 

 rous quartz-veins of Australia, Nova Scotia, the Ural, and the Transvaal 

 are post-Cambrian or post-Silurian in age, because they traverse Silurian 

 rocks. In New South Wales, Queensland, and Nova Scotia they are, at 

 least in part, pre-Carboniferous, because the lowest Carboniferous con- 

 glomerate lies on their edges and contains gold derived fi-om them. 



In the Transvaal some of the gold veins are pre-Devonian ; they 

 traverse Silurian rocks with intrusive granite and diorite. Resting on 

 the denuded edges of the Silurian rocks, and at the base of beds believed 

 to be Devonian, is a conglomerate containing gold. The Devonian rocks 

 are themselves traversed by diorite dykes and by auriferous veins. 



These general considerations supply a key by which the possible 

 occurrence of gold in quantity, or rather its probable non -occurrence, 

 may be anticipated. Gold occurs chiefly in quartz-veins in Cambro- 

 Silurian rocks, or in rocks of other ages -which have been, to some extent, 

 altered from their original condition of soft sediment ; but, as a rule, only 

 where these rocks have been invaded by intrusive masses of igneous 

 rocks — sometimes granite, but chiefly diorite and diabase. In ordinary 

 fossiliferous Secondary rocks the occurrence of gold veins is unlikely. 



In all gold-bearing districts disseminated gold may be expected to 

 occur in rock newer than the auriferous veins ; but with rare exceptions 

 it is only where concentrated in gravel that the gold exists in payable 

 quantity. 



Gold generally occurs in quartz-veins in the free state ; but it is often 

 associated with various metallic sulphides — chiefly iron and copper pyrites. 

 Even here it is probably in a free metallic state, but is so finely divided 

 that its extraction is difficult. All vein-gold is subject to loss in stamping ; 

 but the losses in treating gold which occurs with sulphides are often great. 

 Much gold passes away in a finely divided state in the tailings, and there 

 is a further loss in amalgamation in consequence of the gold not present- 

 ing a free metallic surface to the mercury. Then losses sometimes amount 

 to about 70 per cent, of the total gold in the ore ; it is frequently from 

 30 per cent, to 40 per cent. Recent improvements in mining and metal- 



1887. !•■ L 



